IS456 : 2000 Recommendations for Design of Slabs

Effective Span
(a) For Simply Supported Slab

The effective span is taken as smaller of the following:

(i) Centre to centre of supports.

(ii) Clear distance between the supports plus the effective depth.

(b) For Continuous Slab

In a continuous slab, where the width of support is less than 1/12 of the clear span, the effective span should be taken as given in (a) for simply supported slab.

If the supports are wider than 1/12 of the clear span, or 600mm whichever is less, the effective span shall be taken as under:

(i) For end span, with one end free and the other end continuous or for intermediate spans, the effective span shall be the clear span between supports.

(ii) For end span, with one end free and the other end continuous, the effective span shall be equal to clear span plus half the effective depth of slab or clear span plus half the width of discontinuous support whichever is less.

Deflection Control

For slabs, the vertical deflection limits are specified by maximum l/d ratio:

(a) For spans upto 10m

l/d ratio
Cantilever 7
Simply supported 20
Continuous 26

(b) For spans greater than 10 m, the above value may be multiplied by 10/span, except for cantilever, for which exact deflection calculations should be made.

(c) Depending on the area and type of tensile steel the above values may be modified.

(d) For slabs spanning in two directions, the shorter of the two spans shall be used for calculating span to effective depth ratio.

For two way slabs of small spans (upto 3.5m) with mild steel reinforcement, the shorter span to overall depth (given below) ratios may be assumed to satisfy the deflection limits for loading class upto 3000N/m2 .

Simply supported              : 35

Continuous slab                : 40

For high strength deformed bars the values given above should be multiplied by 0.8.

Reinforcement in Slabs
(a) Minimum Reinforcement

The area of reinforcement in either direction in a slab should not be less than 0.15 percent of the total cross-sectional area in case of mild steel reinforcement. In the case of high strength deformed bars, this values can be reduced to 0.12 percent.

(b) Maximum Diameter

The maximum diameter of the reinforcing bar in a slab should not exceed 1/8th of the total thickness of the slab.

(c) Distribution reinforcement

Distribution reinforcement is provided in the longer span of one ay slab. This steel is as per the minimum reinforcement criteria (a) given above. The function of distribution steel are:

(i) To distribute the concentrated loads coming on the slab.

(ii) To protect against shrinkage and temperature stresses.

(iii) To keep the main steel bar in position.

The distribution steel is kept above the main steel and is not provided with hook at the ends.

(d) Spacing of Reinforcement
(1) Minimum Distance between Bars

(i) The minimum horizontal distance between two parallel main bars shall not be less than

  • The diameter of the bar (largest diameter bar is to be considered).
  • 5mm more than the nominal maximum size of coarse aggregate used in concrete.

(ii) The vertical distance between two layers of main reinforcement shall be more than:

  • 15 mm or
  • 2/3rd the nominal maximum size of aggregate
  • Maximum size of the bar
(2) Maximum Distance between Bars in Tension

(i) The spacing of main steel in a slab should not exceed the following:

  • 3 times the effective depth of slab.
  • 300 mm

(ii) The spacing of the bars provided to act as distribution steel (discussed later) or bars provided for preventing temperature and shrinkage stresses shall not exceed the following:

  • Five times the effective depth of slab
  • 450 mm.
(e) Cover

Nominal cover to be provided in a slab is 20mm and the other values of cover for different environmental conditions.

(f) Bent Up Bars

Some of the main reinforcement in slabs are generally bent up near the supports to take up negative moment which may develop due to partial fixity. Generally alternate bars are bent up at a distance of 0.15l (or l/7) from the centre of supports. The bar available at the upper face should be more than l/10 (0.1l) from the centre of support. The reinforcement detailing of one way slab is shown in Fig.

Slab design
(g) Curtailment of Bars

The bars in a slab may be curtailed as per the recommendation in code. But in practice, the bars are bent and not curtailed in slabs.

(h) Shear design

Slabs are safe in shear (nominal shear stress is very low since b is large) therefore no shear reinforcement is provided in slabs except that the alternate bars are bent up near the supports.

Loads on Slab

The various loads coming on a slab are as follows:

(i) Self weight (dead load) of the slab

(ii) Live loads as per use.

(iii) Finishing and partitioned loads.

These loads can be taken from IS code 875 (Part-I).

 

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